1. Field of the Invention:
The present invention relates to a looper shifting device in an overlock sewing machine for looping the end of a cloth.
2. Description of the Prior Art:
There is known a looper shifting device in an overlock sewing machine capable of shifting a looping operation to be carried out by three threads with use of one needle (hereafter referred to as one needle three threads looping operation) to a looping operation to be carried out by two theads with use of one needle (hereafter referrred to as one needdle two threads looping operation) with one touch operation and disclosed in, for example, Japanese Patent Publication No. 60-20031 and Japanese Utility Model Publication No. 63-1984, etc.
These prior art overlock sewing machines comprise an upper looper fixing lever, a spreader having and end supported by the upper looper fixing lever whereby the spreader is turned so that the operative position where the spreader is engaged in the tip end of the upper looper or inoperative position where the spreader is disengaged from the tip end of the upper looper are shifted by one touch operation without detaching the spreader.
However, the prior art looper shifting device in the overlock sewing machine has such drawbacks that firstly, a large space is required since the exposed spreader shall be turned not to hinder the one needle three threads looping operation by one needle, secondly, a quick shifting operation can not be made since a relatively large motion of the fingers is required for the shifting operation between the operation position and the inoperative position in which one needle two threads looping operation can be carried out. Japanese Patent Publication No. 60-20031 discloses a structure to position the spreader by inserting a projection of the spreader into a thread hole of the upper looper so that the spreader can take a stable operative position while Japanese Utility Model Publication No. 63-1984 discloses a structure to position the spreader by inserting the projection of the spreader into a recess of the upper looper so that the spreader can take a stable operation. These accurate positioning operation of the spreader requires a structure of high accuracy which is an obstacle for manufacturing thereof.